1. Field of the Invention
The present invention relates to an optical information recording medium with which information signals can be recorded and/or reproduced by irradiating a thin film formed on a substrate with a laser or other such high-energy light beam. The present invention further relates to a method for manufacturing such an optical information recording medium, and a method and apparatus for recording on this medium.
2. Background Information
Information recording, erasing and rewriting are performed with respect to a phase change type optical information recording medium by utilizing a phenomenon in which a recording material is changed in phase reversibly between a crystalline phase and an amorphous phase. When a thin film made of a chalcogen material or the like is formed on a substrate of the optical information recording medium, it is possible to switch between the amorphous phase and the crystalline phase by using different optical coefficients (refractive index n and extinction coefficient k) to vary the irradiation conditions of the localized heat produced by a laser beam. This is already common knowledge, and there have been a great deal of research and development and commercial application of so-called phase-changeable optical information recording media.
With a phase-changeable optical information recording medium, the laser output is modulated according to an information signal between at least two power levels (recording level and erasure level) in the irradiation of an information track, which makes it possible to erase existing signals while recording new signals at the same time.
With such an optical information recording medium, in addition to a recording layer, a protective layer composed of a dielectric material with excellent heat resistance, for example, is generally provided on the side of the recording layer closest to the substrate (lower side) and on the side opposite from the substrate (upper side). The functions of such protective layers include preventing thermal deformation of the substrate and evaporation of the recording layer in repeated recording, and enhancing chemical changes and the optical absorbance of the recording layer through an optical interference effect. Providing a reflective layer composed of a metal or alloy material is also standard practice. The functions of a reflective layer include allowing the efficient use of incident light, and raising the cooling rate so as to facilitate amorphitization.
Providing an interface layer between the recording layer and the dielectric layer has also been proposed. Functions of an interface layer include promoting crystallization of the recording layer to improve erasure characteristics, and preventing atomic and molecular inter-diffusion between the recording layer and the dielectric protective layer to improve durability in repeated recording. It is also favorable for the layer to offer good environmental reliability, so that there is no corrosion of or separation from the recording layer.
Providing a material layer between an upper dielectric layer and a reflective layer has also been proposed. The functions of a material layer include improving the erasure rate by adjusting the ratio of optical absorbance between when the recording layer is crystalline and when it is amorphous so that preventing distortion of the mark shape during overwriting, and increasing the C/N ratio by increasing the difference in reflectivity when the recording layer is crystalline and when it is amorphous. It is also preferable for the refractive index to be high and for the layer to absorb light suitably (see Japanese Unexamined Patent Publication No. 2000-215516A, for example).
The basic method for increasing the amount of information that can be stored on a single such optical information recording medium is either to shorten the wavelength of the laser light, or to increase the numerical aperture of the objective lens that condenses the light, thereby reducing the spot diameter of the laser beam and increasing recording surface density. The most popular approach in recent years, as typified by recordable DVDs, is to use an optical system with a wavelength of 660 nm and a numerical aperture (NA) of about 0.6. Furthermore, there have been studies into using blue laser diodes with a wavelength of around 400 nm, which have been nearing the practical stage, and further raising the numerical aperture to about 0.85. Using a numerical aperture this high results in a narrower acceptable margin with respect to the tilt of an optical disk, so it has also been proposed that the thickness of the transparent substrate on the side where the laser beam is incident be reduced from the 0.6 mm of a recordable DVD to about 0.1 mm.
Further, a multi-layer recording medium consisting of a plurality of layers for recording and reproducing information has also been proposed in an effort to increase the amount of information that can be handled with a single medium. With such a multi-layer recording medium, the information layer on the side closest to the laser beam source absorbs light, so recording and reproduction are performed with an attenuated laser beam in the information layer on the side farthest away from the laser beam source, which is a problem in that sensitivity decreases during recording and reflectivity and amplitude decreases during reproduction. Therefore, with a multi-layer recording medium, the information layer on the side closest to the laser beam source must have higher transmissivity, while the information layer on the side farthest away from the laser beam source must have higher reflectivity, reflectivity differential, and sensitivity, and adequate recording and reproduction characteristics must be obtained at limited laser power.
As mentioned above, raising the recording density is important with an optical information recording medium, but raising recording speed is also important in order to be able to handle a large volume of data in a short time. In order to accommodate high speed recording, the crystallization rate of the recording layer must be raised. The crystallization rate is highest with Ge—Sb—Te compositions, which are typical recording materials, and especially with compositions such as GeTe—Sb2Te3.
As discussed above, as new recording and reproduction devices are developed, the trend is for their recording speed to be higher, and media need to be able to keep up with these changes. At the same time, it must also be possible to record at low speed with the same medium in order to ensure compatibility with existing drivers capable only of recording at low speed. Also, from the standpoint of reducing the load on the motor, it is preferable to keep the medium rotating at a constant speed regardless of the radial position on the medium where information is being recorded. Since recording is performed at different linear velocities at the inner and outer tracks of the medium, however, it is necessary for recording on the medium to be possible at both high and low speeds, that is, at or over a specific linear velocity.
In order for a medium to accommodate high speed recording, it is necessary to use a recording layer with a high crystallization rate, as mentioned above. On the other hand, crystallization will be too fast if this recording layer is used for recording at low speed. That is, the problem is that it is difficult to form an amorphous phase and the large marks, so there is a decrease in signal amplitude.
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved optical information recording medium. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.